In a networked client/server system, media content is streamed from a server computer to a client computer. A media file format is used to store data for multiple timeline-altered streams that provides support for switching between the different timeline-altered streams during their presentation. According to one embodiment, a time code stream includes multiple data objects mapping corresponding timeline-altered stream data units to primary stream presentation times, and an index table mapping primary stream presentation times to timeline-altered stream byte offsets.